Feeding Oyster Shells?

I finally got my girls some oyster shells to eat after reading all the posts on here about them, and I just now realized that no one has mentioned how you actually feed them to your hens. So how DO I? Do I just throw them in there in a dish, do I crack them and break them up, or do I turn them into a powder or something? Also is OK for my Scovies to eat them? I dunno if it matters or not but I have a drake and his 3 hens. Do oysters also increase production in ducks?

Offer them crushed oyster shell as a supplement in a dish in a sheltered place. They will eat it, as they need it. For my hens, I simply nailed up a clean tuna fish can on a wall in the run (sheltered from rain/wind). It is at the height of their backs so they don't spill it. They each eat whatever they need - some days more, some less. This way you don't have to worry about which of your breeds need more/or less calcium, or for hens that are molting and definitely don't want the extra calcium during the molt.

By the way, are you using whole shells?? Oyster shell is available at any feedstore, already crushed to the size hens can handle.

ETA: I don't have ducks, but assume they need it too cuz they make eggs. Also, if you need to crush some whole OS, I checked my bag and the size of the pieces are about 1/4".